<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <style>
       .parent {
            width: 300px;
        }
       .width-100 {
            width: 100%;
            background-color: pink;
            margin-bottom: 10px;
        }
    </style>
</head>
<body>
    <div class="parent">
        <div class="width-100">width: 100%</div>
        <div class="width-100">width: 100%</div>
    </div>
</body>
</html>